Machine tools with an automatic tool changer, in particular CNC drills, are widely used in the field of machine tools. Such automatic tool changers are known, for example, from Japanese Unexamined Patent Publication No. 2-15935 and from Japanese Unexamined Patent Publication No. 2-48146.
A prior art automatic tool changer, as shown in FIG. 16, comprises a turret 74 holding a plurality of tools positioned on its peripheral. When a tool-change command is executed, firstly, orientation is carried out. In orientation, the angular position of a key on a spindle, on which a tool (or tool holder) 70 is mounted, is adjusted to a given angular position. Sequentially, a spindle head 60 is raised with the angular position of the spindle maintained. As the spindle head is raised, a cam 62 is engaged with a lever 64, then the lever 64 is rotated by the cam 62. The rotation of the lever 64 is transmitted to a crank 68 through a link 66 provided on an end of the lever 64. Thus, the crank 68 rotates (in the following, the rotation of the crank and the turret is referred to as a swing movement), and a grip 72 arranged on the periphery of the turret 74 approaches the tool holder 70 mounted on the spindle. Then, the grip 72 engages with the tool holder 70 and grasps it. Thereafter, the spindle head 60 is further raised so that the tool holder 70 is removed from the spindle. When the tool holder 70 removed from the spindle, an indexing of the turret 74 is carried out by the rotation of the spindle with a spindle gear 76 being engaged with a turret gear 78. Thus, a desired tool is selected from among the tools held on the turret 74. Sequentially, the spindle head 60 is lowered and the turret gear 78 is disengaged from the spindle gear 76, during which another orientation is carried out in order to correct the discrepancy of the angular position of the spindle, since the angular position of the spindle may be different from the given angular position due to the rotation of the spindle for indexing the turret. When the spindle head 60 is lowered, the tool holder grasped by the grip 72 is mounted on the spindle and the tool change operation is completed.
The conventional automatic tool changer generates shock due to the swing movement of the turret 74 when the grip 72 grasps the tool holder mounted on the spindle or the tool holder grasped by the grip 72 is mounted on the spindle. In order to reduce the shock, in the conventional automatic tool changer, the feed velocity of the vertical movement of the spindle head is reduced when the shock is generated. Thus, the conventional automatic tool changer has a disadvantage that it takes long time for a tool change because of the reduction of the feed velocity.
Furthermore, in order to remove the tool holder from the spindle, in the conventional automatic tool changer, the spindle head 60 must be raised higher until the tool holder is completely removed after the grip grasps the tool holder mounted on the spindle. On the other hand, the spindle head must be lowered until the tool holder is completely mounted on the spindle when the tool holder is mounted on the spindle. Therefore, a long feed length is required in the conventional automatic tool changer. Thus, the conventional automatic tool changer has the further disadvantages that the machine tool is high and the time for tool change is long.
In the prior art, the turret is driven by the spindle, the speed of which is reduced to about 30 rpm without a speed reducer between the turret and the spindle. However, the electric motor, typically a spindle motor, used as an drive source for such an automatic tool changer has a characteristic that the generated power is proportional to the speed of the motor when the motor rotates at a speed under 1500 rpm, that is, the generated torque is constant under 1500 rpm. Therefor, if the turret is driven by the spindle which is rotating at 30 rpm then the power is not enough for a high-speed tool change.
It is obvious that this problem can be solved by providing a compact speed reducer within the turret. The speed reducer within the turret does not require high accuracy or high transmission efficiency for indexing the turret during a tool change.
There are various known speed reducers, such as a multiple gear box, a worm gear device, a bevel gear device, a planetary gear (epicyclic gear) device, a roller gear cam device, and the others. The multiple gear box has high transmission efficiency. However, it has a plurality of gears. The worm gear device has high reduction ratio. However, it has an input axis and an output axis arranged at right angle to each other. Thus, the above two devices tend to have relatively large dimensions. A device having a combination of bevel gears and a device using planetary gears cannot provide the high reduction ratio required for the automatic tool changer. The roller cam gear device can provide a high reduction ratio and high accuracy without back-lash. However, it is a large and expensive device since it has a plurality of axes and complicated roller grooves must be formed on the drive axis thereof.
Therefore, conventional speed reducers are too complicated and large to be arranged within the turret. It is also difficult t o reduce the time required for a tool change by providing the desired reduction ratio. That is, it is difficult to increase the velocity of the turret during a tool change by the conventional speed reducer. Furthermore, from a viewpoint of a cost, because of the high quality, the conventional speed reducer is not suitable as a speed reducer adapted to drive the turret of an automatic tool changer.